The present invention relates to a process of electro-plating a coating of a metal or an alloy onto a substrate employing an electrolytic cell as in conventional electro-plating but which does not use a conventional electroplating regime.
The deposition of metal or alloy coatings by electroplating techniques is well known in the art. Electro-plating is typically carried out at voltages of from 5 to 20 volts DC, current densities of from 0.2 to 60 amp/dm2 (more typically 1-10 amp/dm2) temperatures in the range of from 10 to 90xc2x0 C. and under conditions where the workpiece is completely immersed in the electrolyte solution. It is also common to use sacrificial anodes of the same composition as the plating metal for the purpose of maintaining the electrolyte concentration during the process.
The electro-plating process is relatively slow, typically depositing metal at a rate of from 1 to 5 micrometres/minute in thickness. The electro-plated layer is normally dense (solid and without voids), while its surface finish is normally smooth and reflective towards light. The thickness of electro-plated layers can have almost any value depending upon the time and current density employed. Typical thicknesses would be in the range of from 10 to 50 micrometres.
In xe2x80x98brushxe2x80x99 plating processes, such as the xe2x80x98Sifcoxe2x80x99 process (Sifco Industries Inc. of Cleveland, Ohio, USA), higher rates of electro-deposition of up to 20 micrometres in thickness/minute can be attained under favourable conditions. This process uses a porous pad or xe2x80x98brushxe2x80x99 saturated with the electrolyte at high metal concentrations, the pad occupying the space between the anode and the cathode (workpiece). The workpiece is therefore not immersed in the electrolyte as in conventional electro-plating processes. Voltages of from 6 to 20 v are generally employed.
High speed electro-plating can be carried out at very high current density. For example, a deposition rate of 150 micrometres/minute is reported for an iron plating from a Fe(NH2SO3)2 solution at a current density of 690 amp/dm2 (Electro-plating Engineering Handbook [Ed. L. J. Durney], Van Nostrand Reinhold, 1984, 4th Edition, pages 767-771).
However, these known processes of electroplating are not suitable for producing intentionally rough or porous coatings which may be desired, for example to provide better mechanical keying if a further coating, for example of paint or plastic, is to be applied. In some circumstances also it may be desirable to deposit a metal coating at a much faster speed than is feasible using known electro-plating process at normal current densities (below 100 amp/dm2). The present invention addresses these matters.
In PCT Application No.PCT/IB96/00876, the present inventors describe a process for cleaning and metal-coating electrically conducting surfaces which employs one or more anodes which are made of the metal or metals to be deposited. The method involves transferring metal from the anode or anodes to the workpiece which forms the cathode in an electrolytic cell. The process is operated in a regime in which the DC current decreases or remains substantially constant with increasing voltage and in which discrete gas bubbles are formed at the workpiece. The electrolyte is sprayed or jetted on to the surface of the workpiece through one or more holes in the anode. Although the workpiece may be immersed in the electrolyte, it is preferred that it is not. In the prior Patent Application it is not necessary that the electrolyte should contain any soluble salt or compound of the metal being deposited, since the metal which is deposited onto the surface of the workpiece is transferred from the anode or anodes, which must be of the same composition as the metal or metals to be deposited. The process described in the present Application is distinguished from the said earlier Application in that (a) it involves the conventional deposition of metal from the electrolyte solution and (b) it is carried out using an anode which may be made from any electrically conducting material.
Electro-plating without immersion of the workpiece is known in the xe2x80x98brushxe2x80x99 plating process as mentioned above and is also taught, for example, in CA-A-1165271 in which the electrolyte is pumped or poured through a box-shaped anode with an array of holes in its base so as to impinge on the workpiece situated or moving below the anode-box. The benefit of this arrangement is that only one side of the workpiece is plated instead of the whole, as would occur in a bath plating method. It also avoids the use of a consumable anode.
With regard to operation in a plasma or spark-discharge regime, there are a number of patents in which the purpose is solely to clean a metal surface. Examples are GB-A-1399710, SU-A-1599446, SU-A-1244216 and GB-A-1306337.
With regard to coating processes, micro-arc processes have been described for the deposition of oxide and silicate coatings on metals. In these processes the coatings which are deposited are non-metallic and take place at the anode, not the cathode as in the present invention. (See for example, U.S. Pat. No. 3834999; A. V. Timoshenko et al., Protection of Metals, Vol. 30, No. 2, 1944, pp 175-180).
We have now developed an electro-plating method in which metal is deposited from an electrolytic solution as in a conventional electroplating process, but in the presence of an electrical plasma arc discharge.
According to the present invention there is provided an electrolytic process for metal-coating the surface of a workpiece of an electrically conductive material, which process comprises:
i) providing an electrolytic cell with a cathode comprising the surface of the workpiece and an anode;
ii) introducing an electrolyte comprising an aqueous solution containing one or more water soluble compounds of the metal or metals to be deposited into the zone created between the anode and the cathode in a manner such that the cathode is bathed but not immersed in the said electrolyte; and
iii) applying a voltage between the anode and the cathode, characterized in that
iv) an electrical plasma arc is maintained between the anode and cathode during the deposition of the metal-coating onto the surface of the cathode.